1. Field of the Invention
This invention relates to a liquid crystal composition. More particularly it relates to an antiferroelectric liquid crystal composition and a light-switching element using the composition. Further, the present invention also relates to a process of reducing the threshold voltage of the antiferroelectric liquid crystal composition.
2. Description of the Related Art
Liquid crystal compositions have been broadly used as a display material. Most liquid crystal display elements are those of TN display mode and utilize nematic phase. TN display modes used for liquid crystal display are roughly classified into two areas. One of them is an active matrix mode having a switching transistor fixed onto each pixel. An example thereof uses a thin film transistor (TFT) and its display grade has reached a level matching that of CRT display, but it is difficult to make the size of the pictorial surface larger and also the cost is high.
The other mode is supertwisted nematic (STN) mode. This mode has been improved in the contrast and the width of viewing angle as compared with conventional, simple matrix mode, but the display grade has not yet reached the level of CRT display. However, the cost is not so high. Thus, these two modes have merits and demerits when the display grade and production cost are taken into consideration.
About ten years ago, a display mode using a ferroelectric liquid crystal (abbreviated to FLC) has appeared in expectation of solving the problems of the above two modes. At present, the liquid crystal referred to merely as FLC indicates a surface-stabilized ferroelectric liquid crystal (abbreviated to SSFLC). This SSFLC has been proposed by N. A. Clark and S. T. Lagewall (see Appl. Phys. Lett., Vol. 36, page 899 (1980)). Since then, it has been referred to as a liquid crystal display of the next generation and various researches and developments have been advanced. The reason is that the ferroelectric liquid crystal element has the following specific features:
(1) quick response PA1 (2) exhibition of memory properties and PA1 (3) broad viewing angle. PA1 (i) inferior switching and ghost effect, PA1 (ii) inferior stability to mechanical shock, PA1 (iii) reduction in the brightness and contrast due to occurrence of chevron structure; etc. PA1 (a) The liquid crystal element has a high driving voltage. PA1 (b) The temperature-dependency of the hysteresis characteristics is large.
Due to these specific features, a SSFLC mode is expected to produce a potential high density display which is very interesting as a display device. However, as the researches have been advanced, the following problems to be solved have been revealed:
As a method for solving these problems of SSFLC, an antiferroelectric liquid crystal display element has been noted. The antiferroelectric liquid crystal has been found by Furukawa et al for the first time in terms of SY* phase (see Ferroelectrics, Vol. 85, page 451 (1988)). Thereafter, it has been advocated by Chandani et al that the above phase is antiferroelectric (see Japanese J. of Appl. Phys., vol. 28, page 1265 (1989)), and this view has now been generally accepted.
The antiferroelectric liquid crystal display element utilizes a tristable switching in the antiferroelectric phase. The tristable switching refers to a switching among three states of the bistable state in the ferroelectric state at the time of impression of electric field and an antiferroelectric state at the time of no impression of electric field.
The specific features of the antiferroelectric liquid crystal element consist in that when transition is effected between the antiferroelectric state and the ferroelectric state, steep threshold value characteristics and an optical hysteresis having a broad width are exhibited, and by utilizing the specific features, it is possible to expect a display having a viewing angle to an extent similar to that of SSFLC and also having a high display contrast.
As to the antiferroelectric phase, those of chiral smectic C phase (abbreviated to SC* phase) and chiral smectic I phase (abbreviated to SI* phase) have been reported at present, but an antiferroelectric phase of chiral smectic F phase, too, may be existent in the aspect of its structure. The tristable switching is possible in whatever phase, as far as the phase is an antiferroelectric phase. However, in view of the response rate, an antiferroelectric phase of SC* phase (abbreviated to SC.sub.A * phase) is preferred. In 1989, an antiferroelectric liquid crystal display has been experimentally prepared by Yamawaki et al (see JAPAN DISPLAY, '89 Collected Preprints 3--3, page 26), and it has come to be quickly recognized.
However, the current antiferroelectric liquid crystal materials still yet have the following problems:
The high driving voltage is due to the fact that the antiferroelectric liquid crystal composition has a high threshold voltage. The threshold voltage referred to herein means an electric field intensity to initiate a phase transition from the antiferroelectric phase to a ferroelectric phase by imparting a voltage to an antiferroelectric liquid crystal. The mechanism of occurrence of the threshold voltage has not yet been clarified. Further, no investigation on factors having an influence upon the threshold voltage has ever been reported other than the one made by the present inventors (see the 17th Liquid Crystal Symposium, Collected, Preprints, page 260 (1991)).
The present invention is intended to solve the problem described in the item (a) among the above items, and its first object is to reduce the threshold voltage at the time of transition from the antiferroelectric phase to a ferroelectric phase accompanying impression of an electric field, in a SC.sub.A * phase, and its second object is to provide a light-switching element actuated by a low driving voltage.